Liquid crystal display and backlight adjusting method

ABSTRACT

A liquid crystal display apparatus and backlight adjustment method are provided.  
     Backlight luminance sensors  111 A to  111 D are disposed in the vicinity of four outer corners of an effective screen of an LCD panel  121 . Each of the backlight luminance sensors  111 A to  111 D detects the luminance of each of three primary colors. A backlight unit is composed of a three-primary LED array and a light diffusion unit. Transistors of the backlight luminance sensors and transistors of a pixel portion are formed on the same substrate in the same process. When a transistor is irradiated with backlight in its sufficient off region, an off current occurs due to light excitation. Since the value of the off current corresponds to the luminance of the rays of backlight that irradiates the transistor, the luminance of the backlight is detected with an output voltage into which the off current is converted. As a result, the luminance of the backlight is kept constant.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent Document No.P2003-408735 filed on Dec. 8, 2003, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display apparatus anda backlight adjustment method.

A liquid crystal display (hereinafter abbreviated as an LCD), which isof non-self emitting type, needs backlight as a light source. Examplesof the backlight are a cold cathode ray tube and a light emitting diode(hereinafter abbreviated as an LED). When an LED is used, a white diodecan be used. However, in a liquid crystal television monitor,three-primary color LEDs of R (red), G (green), and B (blue) are oftenused to improve color reproducibility. By mixing these colors of theseLEDs, white backlight is formed.

When a white LED is used for backlight, since the luminance andchromaticity of the backlight depend on the current that flows in theLED, the duty of on time and off time of the current that flows in theLED is controlled as disclosed in related art (Japanese Patent Laid-OpenPublication No. 2002-324685).

In the related art as shown in FIG. 16, the luminance of backlight foran LCD module is adjusted by controlling current, duty, and so forth. Inother words, an output current value of an LED drive current source 11is controlled by a current value control circuit 12. A switch circuit 14is disposed between the LED drive current source 11 and a white LED 15.The switch circuit 14 is turned on/off with a PWM signal of a PWMgeneration circuit 13. The duty ratio of the PWM signal is controlledwith a control signal supplied from a duty ratio control circuit 16.However, the luminance of the backlight of the LCD module described inthe document deteriorates by aged deterioration or the like.

In the past, when an LCD module was shipped, the luminance of thebacklight was adjusted. The luminance of the backlight was controlled bya thermistor as a temperature detection device. Alternatively, the enduser needed to adjust the luminance of the backlight.

Thus, after the liquid crystal display had been shipped, the ageddeterioration of the luminance of the backlight could not be handled orit was improperly adjusted. Thus, the user needed to adjust theluminance of the backlight.

SUMMARY OF THE INVENTION

The present invention provides in an embodiment a thin type liquidcrystal display apparatus and a backlight adjustment method that do notneed the user to adjust the luminance of the backlight and that can beadjusted with high accuracy.

The present invention provides in an embodiment a liquid crystal displayapparatus having a liquid crystal interposed between two substrates anda backlight as a light source for the liquid crystal, comprising:

-   -   a luminance sensor formed on one of the substrates (this        substrate is referred to as the first substrate), the luminance        sensor and thin film devices as pixels being formed on the first        substrate in the same process, the luminance sensor that detects        the luminance of the backlight; and    -   a control circuit that generates a drive signal that keeps the        luminance of the backlight almost constant on the basis of a        detection signal detected by the luminance sensor.

The backlight includes in an embodiment a light emitting device arrayand a diffusion portion, the light emitting device array being anarrangement of repetition of at least three color light emittingdevices, the diffusing portion that diffuses color rays emitted from thelight emitting device array and generates white light.

Alternatively, the backlight includes in an embodiment a light emittingdevice array, a diffusion portion, and a light guide portion, the lightemitting device array that is an arrangement of repetition of at leastthree color light emitting devices in a line shape, the diffusionportion that diffuses color rays emitted from the light emitting devicearray and generates white light, the light guide portion that equallyguides the color rays emitted from the light emitting device array tothe entire surface of the diffusion portion.

Alternatively, the present invention includes in an embodiment aluminance adjustment method for backlight as a light source of whitelight that is a mixture of rays emitted from an arrangement ofrepetition of at least three-color light emitting devices disposed on aliquid crystal display panel, thin film devices being formed as a screenon the liquid crystal display panel, a luminance sensor being disposedon the liquid crystal display panel, comprising the steps of: detectingluminance of the backlight; generating a drive signal on the basis ofthe detected result at the first step; and driving at least three-colorlight emitting devices with the drive signal generated at the secondstep and keeping the luminance of the backlight almost constant.

According to this aspect in an embodiment, since luminance sensors andpixel transistors of the LCD are formed in the same process, a thin typeLCD panel unit can be produced. In addition, the luminance of thebacklight can be kept constant.

The luminance sensor detects an output voltage into which an off currentdue to light excitation corresponding to luminance of light emitted fromthe backlight is converted in the state that a thin film device thatcomposes the luminance sensor is sufficiently turned off. The liquidcrystal display apparatus further comprises an input signal generationportion that generates an input signal having a repetitive period thatis shorter than a period for which the liquid crystal transmits lightand that the user does not recognize as flickering, the input signalgeneration portion that supplies the input signal to the thin filmdevice that compose the luminance sensor; a sample hold portion thatsample-holds a detection signal of the luminance sensor; and a controlcircuit that generates a drive signal that keeps the luminance of thebacklight almost constant on the basis of a signal sample-held by thesample hold portion. The sample hold portion is formed on the firstsubstrate on which thin film devices are formed.

According to this aspect in an embodiment, when a light insulationportion can not be disposed in a substrate opposite to a substrate onwhich thin film devices are formed or when a frame portion that shieldsluminance sensors cannot be disposed, since a potential that causes thesensor portion to sense the luminance of the backlight for a short timethat the observer cannot recognize and that causes it to appear to beblack in the rest of the time is applied to the sensor portion, even ifthe substrate on which the thin film devices are formed is disposed onthe backlight side, the luminance of the backlight can be detected sothat the observer does not recognize the sensor portion.

The liquid crystal display apparatus according to an embodiment thepresent invention can keep the luminance of the backlight constant evenif the aged deterioration takes place in the liquid crystal displayapparatus.

According to the present invention in an embodiment, the luminancedetection means of the backlight can be disposed on the substrate forthe pixel transistors of the LCD. In addition, the luminance detectionmeans can be formed in the process for the pixel transistors. Thus, thecost of the sensors can be decreased. In addition, since the sensors canbe formed in the LCD module, it can be thinned.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a sectional view showing an outlined structure of an LCDdisplay apparatus, FIG. 1B being a plan view showing an outlinedstructure of the LCD display apparatus.

FIG. 2 is a perspective view showing an outlined structure of an LCDpanel unit.

FIG. 3 is a perspective view showing an outlined structure of an exampleof the LCD panel unit.

FIG. 4 is a perspective view showing an outlined structure of anotherexample of the backlight unit.

FIG. 5 is a schematic diagram showing connections of an equivalentcircuit of one pixel in a write mode.

FIG. 6 is a schematic diagram showing connections of an equivalentcircuit of one pixel in a hold state.

FIG. 7 is a schematic diagram showing connections of an equivalentcircuit of a backlight luminance sensor according to an embodiment ofthe present invention.

FIG. 8 is a schematic diagram showing characteristics of the backlightluminance sensor according to the embodiment of the present invention.

FIG. 9 is a schematic diagram showing an example of the positions of thebacklight luminance sensors according to the embodiment of the presentinvention.

FIG. 10 is a perspective view showing a structure of a backlightluminance sensor portion according to the embodiment of the presentinvention.

FIG. 11 is a perspective view showing a structure of a backlightluminance sensor portion according to another embodiment of the presentinvention.

FIG. 12 is a schematic diagram showing connections of an equivalentcircuit of a backlight luminance sensor according to the otherembodiment of the present invention.

FIG. 13 is a timing chart describing the operation of the otherembodiment of the present invention.

FIG. 14 is a block diagram showing a structure for a process for anoutput voltage of the backlight luminance sensor according to thepresent invention.

FIG. 15 is a block diagram showing details of a part of the structurefor the process for the output voltage of the backlight luminance sensoraccording to the present invention.

FIG. 16 is a block diagram showing an example of a backlight luminanceadjustment apparatus according to related art.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a liquid crystal display apparatus anda backlight adjustment method.

Next, with reference to the accompanying drawings, an embodiments of thepresent invention will be described. Before that, a typical structure ofa liquid crystal display apparatus will be described.

As shown in FIG. 1A and FIG. 1B, a liquid crystal display apparatus 100is composed of an LCD panel unit 101 and a backlight unit 102. Inaddition, a circuit for a control system is also disposed in the liquidcrystal display apparatus 100. These units are housed in a housing 200.Reference numeral 200A represents a frame portion that surrounds ascreen of the housing 200.

As shown in FIG. 2, the LCD panel unit 101 is composed of two substrates103A and 103B that are layered. The LCD panel unit 101 is a thin filmtransistor (TFT) liquid crystal. The TFT is categorized as an amorphoussilicon type and a polysilicon type. The amorphous silicon type uses anamorphous material for a substrate. The polysilicon type uses apolysilicon material for a substrate.

In FIG. 2, reference numeral 103A represents a backlight side substrate.Reference numeral 103B represents a screen side substrate. In the TFTliquid crystal, two substrates are oppositely disposed and a liquidcrystal material is interposed therebetween. One substrate is a TFT sidesubstrate on which TFTs and so forth are formed on a glass substrate.The other substrate is an opposite side substrate on which a colorfilter and so forth are disposed. A backlight luminance sensor, whichwill be described later, is formed on the TFT side substrate.

As the substrate 103A shown in FIG. 2, an opposite side substrate isdisposed. As the substrate 103B, a TFT side substrate is disposed. Therelationship of this arrangement is referred to as pattern A.Alternatively, as the substrate 103A, a TFT side substrate may bedisposed. As the substrate 103B, an opposite side substrate may bedisposed. The relationship of this arrangement is referred to as patternB. The present invention can be applied to any one of thesearrangements.

FIG. 3 shows an example of the backlight unit 102. Reference numeral 104represents a three-primary color LED array. Reference numeral 105represents an optical diffusion unit. The LED array 104 is composed of arepetition of a horizontal array of blue LEDs 106B, a horizontal arrayof green LEDs 106G, and a horizontal array of red LEDs 106R. Thethree-primary colors of the LEDs 106B, 106G, and 106R are diffused bythe optical diffusion unit 105. As a result, the optical diffusion unit105 generates white color backlight.

FIG. 4 shows another example of the backlight unit 102. An LED array 107is composed of red LEDs 106R, green LEDs 106G, and blue LEDs 106B thatare alternately arranged in a line shape. The LED array 107 is disposedon the lower end side of a light guide plate 108. The light guide plate108 equally transmits light of each LED of the LED array 107 to theentire surface of the optical diffusion unit 105. The optical diffusionunit 105 mixes the colors of the light guide plate 108. As a result, theoptical diffusion unit 105 generates white color backlight.

In addition to the light emitting devices of three-primary colors, lightemitting devices of another color may be used to improve the colorreproducibility. The backlight adjustment method according to thepresent invention is performed by controlling drive signals of lightemitting devices of individual colors.

To keep the luminance of the backlight constant, a sensor that measuresthe luminance of the backlight is needed. When the sensor that measuresthe luminance of the backlight is disposed in the structure shown inFIG. 3 or FIG. 4, the sensor needs to be disposed in the vicinity of theoptical diffusion unit 105. When the luminance of the backlight isactually detected, it is preferred to dispose the sensor on the screenside. However, when the sensor is disposed on the screen side, thesensor shadows the screen. Thus, the sensor cannot be disposed on thescreen side.

As a practical arrangement method for the sensor, it may be disposed ona side surface of the optical diffusion unit 105 shown in FIG. 4 or inthe space of the backlight unit 102 shown in FIG. 3 and FIG. 4 ratherthan on the LCD panel unit side. However, when the sensor is disposed onthe side surface of the optical diffusion unit 105, the depth of thehousing 200 increases by the thickness of the sensor.

In the LCD module that is composed of LEDs of three primary colors shownin FIG. 3 and FIG. 4, unless the sensor receives the completely mixedlight, the sensor will recognize incorrect luminance. In other words,when the sensor is disposed on the side surface of the optical diffusionunit 105 or in the space of the backlight unit 102, the sensor needs toreceive the mixed light.

According to the present invention, in consideration of theabove-described point of view, white light which the backlight unit 102emits to the LCD panel is detected. In other words, the sensor thatdetects the luminance of the backlight is disposed in the LCD panel. Thesensor detects light with which the inside of the LCD panel isirradiated. Since the LCD panel is irradiated with designed white lightof mixed colors, the sensor can receive white right of mixed colors.

According to the embodiment of the present invention, the LCD panel unit101 has a sensor that detects the luminance of the backlight. Inaddition, according to the embodiment of the present invention, thebacklight luminance sensor and the pixel portion, namely TFTs, of theLCD panel unit 101 shown in FIG. 5 and FIG. 6 are formed in the sameprocess.

In FIG. 5 and FIG. 6, reference numeral 110 represents a structure ofone pixel of the LCD panel unit 101. Tr represents a pixel transistorthat has the same structure as an MOS-FET. G represents a gate line. Srepresents a source line (also called a data line). Cs represents acapacitor. C represents a Cs line. A gate of the transistor Tr isconnected to the gate line G. A source of the transistor Tr is connectedto the source line S. The capacitor Cs is connected between a drain ofthe transistor Tr and the Cs line C. A pixel electrode is connected inparallel with the capacitor Cs. A liquid crystal capacity Cd existsbetween the drain of the transistor Tr and an opposite electrode A. InFIG. 5 and FIG. 6, the transistor Tr is composed of an N channel typetransistor. Alternatively, the transistor Tr may be composed of a Pchannel type transistor. Next, in the following description, it isassumed that the transistor Tr is composed of an N channel typetransistor.

FIG. 5 shows an equivalent circuit of the pixel 110 in a write state.Since signals are supplied to both the gate line G and the source lineS, the pixel becomes active. A potential of the signal supplied throughthe source line S is written to the pixel through the pixel transistorTr. When the pixel transistor Tr is turned on and a current flowsbetween the drain and the source of the transistor Tr, the liquidcrystal capacitor Cd and the capacitor Cs are charged.

FIG. 6 shows an equivalent circuit of the pixel 110 in the state that aminus potential is supplied to the gate line of the transistor Tr andthe transistor Tr is turned off (hold state). When an off current flowsin the pixel transistor Tr through the gate line G, the pixel transistorTr is turned off. The capacitor Cs, which is an auxiliary capacitor,holds the written signal potential until the next signal is written.

FIG. 7 shows a structure of a backlight luminance sensor 111. Qrepresents a transistor of the backlight luminance sensor 111. A gate ofthe transistor Q is connected to a gate line G. A source of thetransistor Q is connected to a terminal 112. A voltage V_(IN) issupplied to the terminal 112. A drain of the transistor Q is connectedto a terminal 113. An output voltage V_(OUT) is obtained from theterminal 113.

The transistor Q is an N channel MOS type transistor like the pixeltransistor Tr. The transistor Q and the transistor Tr of the pixelportion are formed in the same process on the same substrate. Thetransistor Q has characteristics as shown in FIG. 8. The horizontal axisand the vertical axis of FIG. 8 represent a gate potential and a draincurrent, respectively. In this case, the relationship of (gatepotential=potential of gate line G−potential of V_(IN)) is satisfied.The drain current is a current that flows in the transistor Q, namely acurrent that flow between the terminals 112 and 113.

With the gate potential, the transistor Q always has a sufficient offregion. In the sufficient off region, when rays of backlight irradiatethe transistor Q, an off current Ik (also called a leak current) occursdue to light excitation. The value of the off current Ik corresponds tothe luminance of the rays of backlight that irradiate the transistor Q.Thus, with the output voltage V_(OUT) into which the off current Ik isconverted, the luminance of the backlight can be detected. The channelwidth and so forth of the transistor Q are different from those of thepixel transistor so that the off current of the transistor Q can bedynamically varied.

For example, as shown in FIG. 9, backlight luminance sensors 111A, 111B,111C, and 111D are disposed in the vicinity of four outer corners of theeffective screen of an LCD panel 121. The backlight luminance sensors111A to 111D detect the luminance of each of the three primary colors.As shown in FIG. 3, the backlight unit is composed of the three-primarycolor LED array 104 and the optical diffusion unit 105.

Alternatively, the backlight luminance sensors may be disposed at fourinner corners of the effective screen. The effective screen is a regionin which pixels are disposed. Alternatively, backlight luminance sensorsmay be disposed at more positions than the four corners. When theintegration of LCD panel 121 can be increased using for example lowtemperature polysilicon process, the backlight luminance sensors can bedisposed to individual pixels. In this case, the luminance of thebacklight can be measured from the entire region of the effectivescreen.

FIG. 10 shows a green light detection sensor of one backlight luminancesensor, for example, 111A. In FIG. 10, a TFT substrate 131 is disposedon the backlight side. A TFT and a backlight luminance sensor are formedon the TFT substrate 131. An opposite substrate 132 is disposed oppositeto the TFT substrate 131 with a liquid crystal material (not shown)interposed therebetween. The relationship of this arrangement is patternB shown in FIG. 2. The size of each of the substrates 131 and 132 is thesame as the size of one screen. However, for easy understanding, onlyone backlight luminance sensor is illustrated.

White light Lw emitted from the backlight unit passes through a color(for example a green) filter film 133. The color filter film 133transmits green light Lg. In FIG. 10, the color filter film 133 isspaced apart from the TFT substrate 131. Actually, the color filter film133 adheres to the TFT substrate 131. The color filter film 133 adheresto a light transmission portion of the backlight luminance sensor thatdetects the luminance of green light with which the TFT substrate 131 isirradiated.

In addition, backlight luminance sensors (transistors) (not shown inFIG. 10) that detect the luminance of red light and blue light of thewhite light Lw are disposed.

As shown in FIG. 9, the backlight luminance sensors 111A to 111D aredisposed around or inside the effective screen. In this case, thesebacklight luminance sensors 111A to 111 d need to appear to be a blackimage so that the person who watches the screen, namely, the observer,does not feel that the picture quality deteriorates. If the backlightluminance sensor portions always appear to be white or with threeprimary colors, the observer recognizes them to be bright dots. Thus,the observer seems that the picture quality deteriorates.

Thus, in FIG. 10, at least the opposite substrate 132 that is oppositeto the light transmission portion of the backlight luminance sensor 111Ais a light shield region. In this case, the backlight luminance sensorportion may be shielded with a member made of resin such as a frame ofthe housing of the LCD display apparatus instead of the oppositesubstrate 132. When the backlight luminance sensor portion is permittedto always appear to be white, it is not necessary to light shield it.

FIG. 11 shows another embodiment of the present invention. According tothe other embodiment, an opposite substrate 132 is disposed on thebacklight side. A TFT substrate 131 is disposed on the screen side witha liquid crystal material (not shown) interposed therebetween. Therelationship of the arrangement of the opposite substrate 132 and theTFT substrate 131 is pattern A shown in FIG. 2. The size of each of theTFT substrate 131 and the opposite substrate 132 is the same as the sizeof one screen. However, for easy understanding, only one backlightluminance sensor is illustrated.

A green filter 134 is disposed in a region corresponding to a greenlight detection transistor of the backlight luminance sensor 111A of theopposite substrate 132. White light Lw emitted from the backlight unitpasses through the green filter 134. The green filter 134 transmitsgreen light Lg. The green light Lg passes through a liquid crystalmaterial (not shown) and irradiates a transistor Q of the backlightluminance sensor 111A. Alternatively, light that passes through a colorfilter disposed on the opposite substrate 132 may irradiate thebacklight luminance sensor 111A.

FIG. 12 shows a structure of the backlight luminance sensor 111according to another embodiment. Q represents a transistor of thebacklight luminance sensor 111. A gate of the transistor Q is connectedto a gate line G. A source of the transistor Q is connected to aterminal 112. A voltage VIN is supplied to the terminal 112. A drain ofthe transistor Q is connected to a terminal 113. An output voltageV_(OUT) is obtained from the terminal 113. A liquid crystal capacitor Cdexists between the source of the transistor Q and an opposite electrodeA.

Like the pixel transistor Tr, the transistor Q is an N channel MOS typetransistor. The transistor Q and the transistor Tr of the pixel portionare formed in the same process. As described above, with a gate voltage,the transistor Q always has a sufficient off region. In the sufficientoff region, when rays of the backlight irradiate the transistor Q, anoff current Ik occurs due to light excitement. The value of the offcurrent Ik corresponds to the luminance of the rays of backlight thatirradiate the transistor Q. Thus, with the output voltage V_(OUT) intowhich the off current Ik is converted, the backlight luminance can bedetected.

In FIG. 12, reference numeral 135 represents an opening portion. Theopening portion 135 of the opposite substrate 132 cannot be lightshielded. When the opening portion 135 is light shielded in thearrangement shown in FIG. 11, the rays of backlight do not irradiate thetransistor Q of the backlight luminance sensor. Thus, a sense time thatthe observer cannot recognize is set.

FIG. 13 shows an example of timing at which the luminance of backlightis detected. FIG. 13 shows an input voltage V_(IN), an output voltageV_(OUT), and a gate line potential in the order. The gate line potentialis a minus potential V_(off) that is lower than the threshold voltage ofthe transistor Q. The gate line potential is a level at which thetransistor Q is sufficiently turned off.

In the structure of the connections shown in FIG. 12, the input voltageV_(IN) is applied to the liquid crystal interposed between the TFTsubstrate 131 and the opposite substrate 132. The input voltage V_(IN)sets a white signal (a signal that causes the liquid crystal to transmitlight) and a black signal (a signal that causes the liquid crystal toshield light) on the basis of the potential of the opposite electrode A.Assuming that the sum of the period Tb of the black signal and theperiod Tw of the white signal is a measurement interval, the period Twis sufficiently short in the measurement interval. The measurementinterval depends on the drive method of the LCD and the performance ofthe transistor. In the LCD composed of amorphous transistors, themeasurement interval is preferably in the range from severalmicroseconds to ten several milliseconds. The period Tw of the whitesignal is selected so that the observer is not bothered by a white image(flickering).

When the level of the white signal is applied as the input voltageV_(IN) in the period Tw, the liquid crystal transmits light. As aresult, the green light Lg that passes through the color filter 134irradiates the transistor Q of the backlight luminance sensor 111A. Thetransistor Q generates the detected voltage Vs as the output voltageV_(OUT). With the level of the detection voltage Vs, the luminance levelof the backlight can be detected. The output voltage V_(OUT) containsthe offset voltage Vf.

As shown in the timing chart shown in FIG. 13, the short time Tw thatthe observer cannot recognize is set as a sense time. In the otherperiod, a potential for black is applied to the backlight luminancesensor. According to the embodiment shown in FIG. 11, not only theoptical characteristics of the backlight, but the luminance of thebacklight including the optical characteristics of the LCD color filterdisposed on the opposite substrate 132 can be detected.

In the arrangement shown in FIG. 11, a frame may be disposed around thescreen of the liquid crystal display apparatus on the screen side of theTFT substrate 131 so that the frame shields light.

FIG. 14 shows an example of a structure of a system that processes anoutput signal of a backlight luminance sensor. This system can beapplied to both the foregoing embodiments. For example, detectionvoltages of the backlight luminance sensors disposed at four corners ofthe screen shown in FIG. 9 are supplied to amplifiers 142A, 142B, 142C,and 142D. Output voltages of the amplifiers 142A, 142B, 142C, and 142Dare supplied to latches 143A, 143B, 143C, and 143D. The latches 143A to143D are circuits that latch levels of detection voltages atpredetermined timing defined with latch pulses. The latches 143A to 143Dare composed of for example sample hold circuits.

Output signals of the latches 143A to 143D are supplied to amicrocomputer 144. The microcomputer 144 generates a compensation signalthat keeps the luminance of the backlight constant. The compensationsignal is supplied to an LED controller 145. The LED controller 145generates a drive current to drive a red LED group 146R, a green LEDgroup 146G, and a blue LED group 146B.

The amplifier 142A and the latch 143A have signal paths corresponding torays of three primary colors. Likewise, the amplifiers 142B, 142C, and142D and the latches 143B, 143C, and 143D have signal pathscorresponding to rays of three primary colors.

FIG. 15 shows an example of one signal path of the system shown in FIG.14. The off current of the transistor Q of the backlight luminancesensor varies with light as a variable resistor 147. The backlightluminance sensor is for example a red light sensor. The output voltageV_(OUT) of the backlight luminance sensor is supplied to the latch 143Athrough the amplifier 142 a. The value of the detection voltage of theoutput voltage V_(OUT) is latched by the latch 143A.

An output of the latch 143A is converted into a digital detection signalof for example six bits by an A/D converter 148. The digital detectionsignal of six bits is supplied to a compensation portion 149. A defaultvalue 150 of a luminance level held in a hold portion 150 is supplied tothe compensation portion 149. The default value of the luminance levelcan be freely set.

The compensation portion 149 compares the value of the digital detectionsignal with the default value and repeats an addition or a subtractionuntil they become equal. The compensation portion 149 detects thedifference between the digital detection signal and the default valueand outputs a digital difference signal of six bits. A D/A converter 151outputs the digital difference signal as an analog compensation signal.

The A/D converter 148, the hold portion 150, the compensation portion149, and the D/A converter 151 represent functions as blocks, thefunctions being accomplished by the microcomputer 144 shown in FIG. 14.The analog difference signal is supplied from the D/A converter 151 to adrive current decision portion 152. The drive current decision portion152 detects a drive current. The drive current decision portion 152corresponds to the LED controller 145 shown in FIG. 14.

The drive current decision portion 152 decides a drive current of thered LED group 146R. The red LED group 146R lights with the drivecurrent. In the structure shown in FIG. 15, a drive current of the redLED group 146R is decided with the luminance detected by one backlightluminance sensor. In this case, the luminance of an LED in the vicinityof the backlight luminance sensor is controlled. When the duty ratiocontrol circuit 16 and the PWM generation circuit 13 are disposed asshown in FIG. 16, the output of the D/A converter 151 may be directlyinput to the duty ratio control circuit 16. In this case, the drivecurrent decision portion 152 is omitted.

In the arrangement shown in FIG. 9, by dividing the length and breadthof the screen are divided by two each, four divided regions areobtained. The LED groups of the LED unit are divided so that theycorrespond to the four divided regions. Drive currents formed with thedetection signals of the backlight luminance sensors are supplied toLEDs of the groups. When the output voltage V_(OUT) shown in FIG. 15 isthe output of the backlight luminance sensor 111A (see FIG. 9), the redLED group 146R is an LED group corresponding to the upper right regionof the four divided regions.

The process that correlates the positions of the backlight luminancesensors and the positions of the LED groups is an example.Alternatively, optimum drive currents may be generated by combiningoutput signals of the backlight luminance sensors. For example,compensation signals formed of two luminance sensors may be linearlyinterpolated so as to generate compensation signals of individualportions of the screen.

In the structure shown in FIG. 14, in a device such as a low temperaturepolysilicon, when peripheral circuits such as the amplifiers 142A to142D and the latches 143A to 143D and the TFTs are formed on the samesubstrate, the peripheral circuits may be integrated with an LCD panel.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alternations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

For example, when a white fluorescent lamp is used instead of threecolor LEDs, an AC pulse voltage supplied to the fluorescent lamp may bevaried corresponding to the analog difference signal.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1. A liquid crystal display apparatus having a liquid crystal interposedbetween two substrates and a backlight as a light source for the liquidcrystal, comprising: a luminance sensor formed on one of the substrates(this substrate is referred to as the first substrate), the luminancesensor and thin film devices as pixels being formed on the firstsubstrate in the same process, the luminance sensor that detects theluminance of the backlight; and a control circuit that generates a drivesignal that keeps the luminance of the backlight almost constant on thebasis of a detection signal detected by the luminance sensor.
 2. Theliquid crystal display apparatus as set forth in claim 1, wherein thebacklight includes a light emitting device array and a diffusionportion, the light emitting device array being an arrangement ofrepetition of at least three color light emitting devices, the diffusingportion that diffuses color rays emitted from the light emitting devicearray and generates white light.
 3. The liquid crystal display apparatusas set forth in claim 1, wherein the backlight includes a light emittingdevice array, a diffusion portion, and a light guide portion, the lightemitting device array that is an arrangement of repetition of at leastthree color light emitting devices in a line shape, the diffusionportion that diffuses color rays emitted from the light emitting devicearray and generates white light, the light guide portion that equallyguides the color rays emitted from the light emitting device array tothe entire surface of the diffusion portion.
 4. The liquid crystaldisplay apparatus as set forth in claim 1, wherein the substrate onwhich the thin film devices are formed when viewed from the liquidcrystal side is disposed on the backlight side, at least one luminancesensor being disposed in a screen on which the pixels are formed, alight shield portion being disposed on the other substrate (thissubstrate is referred to as the second substrate) so that the lightshield portion is opposite to the luminance sensor.
 5. The liquidcrystal display apparatus as set forth in claim 1, wherein the secondsubstrate opposite to the first substrate on which the thin film devicesare formed is disposed on the backlight side when viewed from the liquidcrystal, at least one luminance sensor being disposed outside a screenon which the pixels of the thin film devices are formed, and wherein theliquid crystal display apparatus further comprises: a housing thathouses the first substrate, the second substrate, the backlight, and thecontrol circuit and that covers the luminance sensor.
 6. The liquidcrystal display apparatus, wherein the second substrate opposite to thefirst substrate on which the thin film devices are formed is disposed onthe backlight side when viewed from the liquid crystal, wherein theluminance sensor detects an output voltage into which an off current dueto light excitation corresponding to luminance of light emitted from thebacklight is converted in the state that a thin film device thatcomposes the luminance sensor is sufficiently turned off, and whereinthe liquid crystal display apparatus further comprises: an input signalgeneration portion that generates an input signal having a repetitiveperiod that is shorter than a period for which the liquid crystaltransmits light and that the user does not recognize as flickering, theinput signal generation portion that supplies the input signal to thethin film device that compose the luminance sensor; a sample holdportion that sample-holds a detection signal of the luminance sensor;and a control circuit that generates a drive signal that keeps theluminance of the backlight almost constant on the basis of a signalsample-held by the sample hold portion.
 7. The liquid crystal displayapparatus as set forth in claim 6, wherein the sample hold portion isformed on the first substrate on which thin film devices are formed. 8.The liquid crystal display apparatus as set forth in claim 1, whereincolor filters corresponding to at least three color light emittingdevices are disposed on one of the two substrate, wherein the luminancesensors are disposed corresponding to the light emitting devices anddetect the luminance of each of the colors, and wherein the controlcircuit generates drive signals for the light emitting devicescorresponding to the luminance of each of the colors.
 9. A luminanceadjustment method for backlight as a light source of white light that isa mixture of rays emitted from an arrangement of repetition of at leastthree-color light emitting devices disposed on a liquid crystal displaypanel, thin film devices being formed as a screen on the liquid crystaldisplay panel, a luminance sensor being disposed on the liquid crystaldisplay panel, comprising the steps of: detecting luminance of thebacklight; generating a drive signal on the basis of the detected resultat the first step; and driving at least three-color light emittingdevices with the drive signal generated at the second step and keepingthe luminance of the backlight almost constant.
 10. A liquid crystaldisplay method for the luminance adjustment method for backlight as alight source of white light that is a mixture of rays emitted from anarrangement of repetition of at least three-color light emitting devicesdisposed on a liquid crystal display panel, thin film devices beingformed as a screen on the liquid crystal display panel, a luminancesensor being disposed on the liquid crystal display panel of claim 9,comprising the steps of: generates an input signal having a repetitiveperiod that is shorter than a period for which the liquid crystaltransmits light and that the user does not recognize as flickering andsupplying the input signal to the thin film device that composes theluminance sensor; sample-holding the detected signal of the luminancesensor on the basis of the input signal supplied at the first step;generating a drive signal on the basis of the signal sample-held at thesecond step; and driving at least three-color light emitting deviceswith the drive signal generated at the third step so as to keep theluminance of the backlight almost constant.